1. Field of the Invention
The invention relates to a heating device of the light irradiation type, in which a round blank-like substrate fixed by a guard ring, such as a semiconductor wafer or the like, is quickly heated and thus treated, and a holding device for the substrate.
2. Description of Related Art
A round blank-like substrate, such as a semiconductor wafer, is subjected to heat treatment for layer formation, diffusion, baking and the like which comprises rapid heating, holding at a high temperature, and rapid cooling. Using a heating device of the light irradiation type, a temperature increase to at least 1000.degree. C. can be achieved in a few seconds and rapid cooling obtained by stopping the light irradiation.
The temperature in the peripheral area of the semiconductor wafer as a result of heat irradiation from the outer circumferential surface of the semiconductor wafer or for similar reason is lower than in its middle area, even if the semiconductor wafer surface is uniformly irradiated with light. In the case, for example, in which the temperature of the middle area of the semiconductor wafer is 1100.degree. C., the temperature in the peripheral area is about 30.degree. C. lower than in the middle area. In this way, if the middle area and the peripheral area of the semiconductor wafer have a temperature difference, and thus the temperature distribution becomes nonuniform, there are cases in which a dislocation error occurs which is called slip, and in which scrap is produced.
To prevent formation of slip in the semiconductor wafer, a guard ring is used as the measure to prevent formation of the temperature difference between the middle area and the peripheral area of the semiconductor wafer during its heat treatment by rapid heating, holding at a high temperature, and rapid cooling. This guard ring is an annular body which is made of a thin plate of a metal with a high melting point, such as molybdenum, tungsten or tantalum, or of a ceramic, such as silicon carbide or the like. In the inner circumferential area of its circular opening, a semiconductor wafer bearing part is formed which is a drilled-out opening of the annular guard ring.
The peripheral area of the guard ring is supported by a guard ring support frame which is located over the entire periphery of the base plate of the heating device. The semiconductor wafer is arranged such that it is installed in the circular opening of the guard ring. The bearing part of the guard ring holds the semiconductor wafer. Due to the light irradiation, the guard ring itself reaches a high temperature and also heats the opposite outer circumferential surface of the semiconductor wafer by heat radiation, compensating for the heat radiation from the outer circumferential surface of the semiconductor wafer. In this way, the temperature difference between the middle area and the peripheral area of the semiconductor wafer is reduced, the temperature distribution is made essentially uniform, and the formation of slip is prevented.
As was described above, in its circular opening, the annular guard ring which is supported by the guard ring support frame fixes the semiconductor wafer peripherally in its circular opening. The guard ring itself, due to light irradiation, reaches a high temperature. In this way, the heat radiation from the outer circumferential surface of the semiconductor wafer is compensated. To achieve suitable compensation (neither too much nor too little) and to eliminate the temperature difference between the middle area and the peripheral area of the semiconductor wafer, a state is desired in which the guard ring is near the semiconductor wafer and can act more or less as the outer edge area of the semiconductor wafer. This means that it is preferred that the height of the inner circumferential surface of the circular opening of the guard ring is equal to the height of the outer circumferential surface of the opposite semiconductor wafer, the thickness of the inner circumferential surface of the circular opening of the guard ring is likewise equal to the thickness of the semiconductor wafer and also the heat capacity per unit of area is as great as the heat capacity of the semiconductor wafer per unit of area. When one such guard ring is used, during light irradiation, a temperature change of the guard ring exactly following the temperature change of the semiconductor wafer can be achieved. Therefore, the heat radiation from the outer circumferential surface of the semiconductor wafer can be exactly compensated and a uniform temperature distribution in the semiconductor wafer obtained. One such guard ring is disclosed for example in the Japanese patent disclosure document HEI 9-22879.
Conventionally, the outside diameter of a semiconductor wafer has been for the most part 200 mm (8 inches). But recently, it has been becoming larger and larger, and is gradually approaching 300 mm (12 inches). Furthermore, there are cases in which a semiconductor wafer with a diameter of less than 200 mm is used. Therefore, for a heating device of the light irradiation type, a device is also desired which can be used for heating of semiconductor wafers with different outside diameters.
It is necessary that the guard ring support frame, in any case, be so large that it can support a guard ring for the largest semiconductor wafer when several different semiconductor wafers with different outside diameters are to be processed in the same device.
The inscribed circle of the guard ring support frame must therefore be as large as the outside diameter of the guard ring for the largest semiconductor wafer. It is therefore necessary that the outside diameter of a guard ring for a semiconductor wafer with a small outside diameter be made as large as the outside diameter of a guard ring for the largest semiconductor wafer. In this way, the width of an annular guard ring (difference between the outside radius and the inside radius of the guard ring) becomes greater. A guard ring with a width of about 80 mm is needed for a semiconductor wafer with a diameter of 200 mm so that a semiconductor wafer with a diameter of 200 mm can also be supported by a guard ring support frame which supports a guard ring for a semiconductor wafer, for example, with a diameter of 300 mm.
The outer circumferential surface of the guard ring is in contact with the guard ring support frame. However, since the guard ring and the guard ring support frame have different shapes, even if they are produced from the same material, the heat capacity per unit of area is different, the heat capacity of the guard ring support frame being greater. The heat of the guard ring is therefore transported via the contacting surfaces to the guard ring support frame. Furthermore, heat is radiated from the outer circumferential surface of the guard ring. The temperature of the outer circumferential surface of the guard ring therefore becomes lower even if light is radiated such that the semiconductor wafer and the guard ring have the same temperature. In this way, the temperature distribution becomes nonuniform. The heat in the area of the guard ring in the vicinity of the semiconductor wafer is transported to the outer circumferential surface; this influences the relation between the heat radiation and the compensation which take place between the semiconductor wafer and the guard ring.
When the output of a heating lamp is controlled to make the temperature of the semiconductor wafer uniform, therefore the temperature distribution in the guard ring must be considered, making control difficult.
When the width of the guard ring is large, high thermal stress occurs in the guard ring as a result of the temperature difference between the area in the vicinity of the semiconductor wafer and the outer circumferential surface. There are therefore cases in which distortion of the guard ring, or in extreme case fracture of the guard ring, occurs.